Eosinophilic pulmonary diseases are a heterogeneous group of disorders characterized by the accumulation of eosinophils in alveolar spaces, the interstitium, or both. Peripheral blood eosinophilia is also common. Known causes of eosinophilic pulmonary disease include
Drug-induced pneumonitis (eg, caused by therapeutic drugs, such as antibiotics, phenytoin, or L-tryptophan)
Infections (especially helminthic infections)
Inhaled toxins (eg, recreational drugs, such as cocaine)
Systemic disorders (eg, eosinophilic granulomatosis with polyangiitis [formerly Churg-Strauss syndrome])
Often the cause is unknown.
The two primary eosinophilic pulmonary diseases of unknown etiology are
Hypereosinophilic syndrome, a systemic disease affecting multiple organs, is discussed elsewhere.
Löffler syndrome, a syndrome of fleeting pulmonary findings and peripheral blood eosinophilia, is another eosinophilic pulmonary disease.
(See also Overview of Interstitial Lung Disease.)
Diagnosis is based on demonstration of opacities on chest imaging and identification of eosinophilia (> 450/microL [0.45 × 109/L]) in peripheral blood, bronchoalveolar lavage fluid, or lung biopsy tissue. However, pulmonary eosinophilia may occur in the absence of peripheral eosinophilia. Pulmonary opacities on chest imaging associated with blood eosinophilia are sometimes called PIE (pulmonary infiltrates with eosinophilia) syndrome.
Eosinophils are primarily tissue-dwelling and are several hundred–fold more abundant in tissues than in blood. Consequently, blood eosinophil numbers do not necessarily indicate the extent of eosinophilic involvement in affected tissues. Eosinophils are most numerous in tissues with a mucosal epithelial interface with the environment, such as the respiratory, gastrointestinal, and lower genitourinary tracts. Eosinophils are not present in the lungs of healthy people, so their presence in tissue or bronchoalveolar lavage fluid (> 5% of differential count) identifies a pathologic process.
Eosinophils are exquisitely sensitive to corticosteroids and completely disappear from the bloodstream within a few hours after administration of corticosteroids. This rapid disappearance from the blood may obscure the diagnosis in patients who receive corticosteroids before the diagnostic assessment is instituted.